As a blue light-emitting phosphor placed in a visible light-emitting apparatus for giving an emission of a visible light in response to irradiation of ultraviolet rays (wavelength: 254 nm) produced by discharge of mercury vapor in a fluorescent lamp, a blue light-emitting aluminate phosphor (BAM: (Ba,Eu)O.MgO.5Al2O3) is generally employed. However, the blue light-emitting BAM phosphor is apt to decrease its emission strength with the passage of time.
On the other hand, there has been known a blue light-emitting silicate phosphor having a formula of 3(Me,Eu)O.aMgO.bSiO2, in which Me is an alkaline earth metal selected from the group consisting of Sr, Ca and Ba, “a” is a number in the range of 0.9 to 1.1, and “b” is a number in the range of 1.8 to 2.2. However, it is known that the blue light-emitting silicate phosphor generally gives an emission of strength lower than that of the emission produced by the blue light-emitting aluminate phosphor (BAM).
JP 2006-70187 A (Patent Publication 1) refers to a blue light-emitting silicate phosphor and describes that the strength of an emission given by the silicate phosphor in response to irradiation of vacuum ultraviolet rays of a wavelength of 147 nm is enhanced by incorporating a such element as Cu, Ga, Ge, As, Ag, Cd, In, Sn, Sb, Au, Hg, Tl, Pb and Bi into the silicate phosphor. In the working examples given therein, a blue light-emitting silicate phosphor is produced by calcining a powdery mixture comprising SrCO3 (MeO source), MgCO3 (MgO source), SiO2 (SiO2 source), Eu2O3 (EuO source), and NH4Br (flux).
JP 2007-314644 A (Patent Publication 2) describes that a blue light-emitting silicate phosphor in which a portion of Mg is replaced with a metal selected from Groups 5 and 6 shows an improvement of emission strength, after the phosphor is irradiated with vacuum ultraviolet rays of a wavelength of 146 nm at a lapse of one hour and 24 hours. In the working examples given therein, a blue light-emitting silicate phosphor is produced by calcining a powdery mixture comprising a combination of barium carbonate and strontium carbonate (MeO source), basic magnesium carbonate (MgO source), silicon dioxide (SiO2 source) and Europium oxide (EuO source).